1. Field of the Invention
The present invention relates to an image input device such as a digital camera, and more particularly relates to an image input device intended to improve a light quantity adjusting technology for acquiring a high quality image.
2. Description of the Related Art
In an image input device such as a digital camera which uses image forming optical means which forms an object image by means of an optical system typified by lenses, when the aperture is reduced for imaging an object, the image quality of the object image degrades due to a diffraction phenomenon of light a periphery of the aperture. To address this problem, a high-class camera often employs light reduction means which uses a neutral density (ND) filter or the like to reduce the transmitting light quantity, thereby reducing light without reducing the aperture, resulting in eliminating the necessity of reducing the aperture beyond a certain degree.
On the other hand, in imaging on various cameras, imaging effects by means of the aperture are utilized for artistic expression. The imaging effects by means of the aperture include reducing the aperture to increase the depth of field, thereby focusing in a wide range, resulting in a pan focus-like expression, or conversely, increasing the aperture to decrease the depth of field, thereby defocusing foreground and background other than an intended object, resulting in an expression which shows off the intended object. Particularly, for users referred to as high-end users who freely use high-level imaging techniques, expressions employing these imaging effects by means of the aperture are usual, and they want cameras on which the imaging effects by means of the aperture are available. In order to offer these effects, high-class cameras often employs a multistage aperture which can control the aperture at multiple stages.
To attain the above two requirements, it is efficient to use a multistage aperture which employs an ND filter together with a general multistage aperture. FIG. 2 shows an exterior of an example of a shutter unit on which a multistage aperture together with an ND filter is installed. The multistage aperture 13 generally includes multiple aperture blades, and is configured to employ a pulse motor, for example, to drive the respective aperture blades via a cam, thereby obtaining a desired aperture diameter in FIG. 2. If a pulse motor is employed as a drive source for the multistage aperture 13, the aperture diameter is controlled stepwise, and is generally designed so that the aperture diameter is changed by ½ AV (aperture value, used in a so-called apex system) or ⅓ AV per pulse. The ND filter 15 is generally constructed by affixing a filter member to blade members serving as support members, and controls the transmitting light quantity by moving the blade members forward and backward with respect to the optical axis, thereby inserting/extracting the filter member to/from the optical axis.
In this case, though a large ND filter which can adjust the light quantity in the full aperture may be employed, the filter member constituting the ND filter is expensive, and the ND filter is preferably constructed by a more or less small filter member.
In other words, as long as an exposure control characteristics indicated as exposure charts (refer to FIGS. 3, 4, 5, 6, 8, and 10) can be constituted, an ND filter is to be configured using a smaller filter member which can be applied to a state starting from the aperture is more or less reduced. If an ND filter using a smaller filter member is employed, a mechanism which holds and drives the ND filter can be compact. The example shown in FIG. 2 has a configuration in dimensional relationship that the ND filter 15 can be used when the multistage aperture 13 reduces the aperture value by 1 AV or more from the full aperture.
With the above-described multistage aperture simultaneously employing the ND filter used for high class models, the ND filter can be used at any aperture value ranging from a full aperture state or a nearly full aperture state to a maximally reduced aperture state. As a result, if a certain light reduction quantity is required, there is a case where the light reduction is realized by means of only the multistage aperture and by the multistage aperture along with the ND filter. FIG. 9 shows an example of this case. The reduction by the multistage aperture and the reduction by the multistage aperture along with the ND filter are both available in grayed entries in FIG. 9. FIG. 10 shows this as an exposure chart.
Both the case where the ND filter is not inserted and the case where the ND filter is inserted are present at six stages from F5 to F9 in FIGS. 9 and 10. The full aperture is represented as a circled “1”, and the minimum aperture is represented as circled “12” in FIGS. 9 and 10, resulting in 12 stages, and the respective circled numbers represent the stages of the multistage aperture (different from “aperture stages”, which are referred to as “nth step of the aperture”. “Aperture stage” generally corresponds to an AV value).
Though cases where the ND filter is simultaneously used for the 9th stage (circled “9”) to the 12th stage (circled “12”) of the multistage aperture are shown in FIGS. 9 and 10, since it is sufficient that an actual camera can be applied up to LV 18 (LV: Light Value, used for the apex system), these four stages of the light reduction states are not used. Moreover, in the example of the multistage aperture shown in FIG. 2, since the ND filter 15 can be used in a state with an aperture value reduced from the full aperture by 1 AV or more, the simultaneous use of the ND filter is not applied to the first stage (circled “1”) to the third stage (circled “3”), and since the aperture is not reduced so much in this range, the influence of the degradation of the image quality due to the diffraction is seldom present, and the simultaneous employment of the ND filter is not necessary in terms of the image quality.
If the same light reduction state with respect to a certain luminance of the object is realized by the aperture and the aperture along with the ND filter as described above, there poses a problem of determining which of them is more proper to use. The advantages and disadvantages of the light reduction only by the multistage aperture and the light reduction only by the ND filter are compared in FIG. 12.
The most important thing for image input devices such as a camera is how to acquire a better image quality, and a problem to be considered with this respect is the above-described problem of the image degradation upon a small aperture. The main reason for employing the ND filter as the light reduction means is to address the degradation of the image quality upon the small aperture. The employment of the ND filter eliminates the necessity of a small aperture, thereby eliminating the degradation of the image quality. FIG. 11 shows an example of a relationship between the aperture and an image forming capability (MTF) corresponding to the image quality. Thought the MTF can be maintained to a somewhat proper value from the full aperture to an aperture reduced by approximately 1.5 stages as shown in FIG. 11, if the aperture is reduced further, the MTF apparently degrades. In order to prevent the MTF from degrading, it is preferable to additionally employ the light reduction by the ND filter so as to eliminate the necessity of using a small aperture.
On the other hand, the reason for employing the multistage aperture is to control the depth of field according to the intention of a photographer. It is necessary to reduce the aperture by means of the multistage aperture thereby increasing the depth of field in order to focus in a wide range from a near view to a distant view.
Moreover, there is a difference between the light reduction by the multistage aperture, and the light reduction by the ND filter in terms of problems of ghosts and a reduction of quantity of light on periphery. In terms of the ghosts, the light reduction by the ND filter, which inserts an ND filter, is disadvantageous due to an increase of reflecting surfaces, and if ghosts are blocked by an aperture, the ghosts are reduced by reducing the aperture by means of the multistage aperture. In terms of the reduced quantity of light on periphery, in general, if the aperture is reduced by control, though the light quantity becomes uniform, and the reduction of the quantity of light on periphery is improved, the ND filter does not improve the reduction of the quantity of light on periphery.
Moreover, in terms of power consumption, there is a difference between the light reduction by the multistage aperture, and the light reduction by the ND filter. In general, the multistage aperture is often driven by a pulse motor, and the pulse motor including two coils tends to consume a more electric power. The ND filter moves only between an inserted position and a retracted position with respect to the optical axis, and it is thus possible to employ a moving magnet including a single coil, resulting in a small amount of the power consumption. The best strategy to reduce the power consumption is to reduce the movement as much as possible for both of them.
In view of the foregoing technical background, an imaging device which provides control to selectively switch to the optimal camera control method according to the advantages and disadvantages of the light reduction by the multistage aperture and the light reduction by the ND filter is proposed in Japanese Patent Laid-Open No. 2003-134393, for example. More specifically, the imaging device according to Japanese Patent Laid-Open No. 2003-134393 includes exposure control means which controls aperture control means and transmittance control means, which employs an ND filter or the like, in combination, and the exposure control means includes a first operation mode which controls the aperture control means and the transmittance control means preferentially for the depth of field. Moreover, the exposure control means includes, in addition to the first operation mode, a second operation mode which controls the aperture control means and the transmittance control means preferentially for resolution, and provides control for switching between the first operation mode and the second operation mode according to imaging conditions.
The present applicant improves the system disclosed in Japanese Patent Laid-Open No. 2003-134393, and proposes a technology which minimizes the use of the control means, which changes the transmittance by means of the ND filter or the like, as Japanese Patent Laid-Open No. 2007-114283.
The applicant focused on the problem of ghosts which are generated by the ND filter in order to further improve prior art thereby acquiring image which more suits an intention of a photographer.
An item “GHOSTS” in FIG. 12 is first studied in detail.
If the aperture is reduced, ghosts can be reduced compared with the case of the full aperture in the control by the aperture while ghosts may increase and the image quality may degrade since the reflecting surfaces are increased by the insertion of the ND filter in the control by the ND filter. Especially, if a surface perpendicular to the optical axis such as an ND filter is present close to the position of the aperture, there occurs reflection between the ND filter and some surfaces close to an imaging element such as a CCD (Charge Coupled Device) such as a low-pass filter, a cover glass plate of the imaging element, and the imaging surface itself similarly perpendicular to the optical axis, thereby often causing ghosts.
FIG. 13 is a schematic view showing a principle of the generation of an example of ghosts due to the ND filter. A light beam returns toward an aperture after reflected by a low-pass filter, a cover glass plate, an imaging element, or an imaging plane itself as a first reflecting surface, is reflected again on a rear surface of an ND filter as a second reflecting surface, and reaches the imaging plane resulting in a ghost. This is a major principle of the ghosts caused by the ND filter. The fact that the first reflecting surface is a flat surface close to the imaging plane implies that light on an acquired image plane is a cause of the ghosts, which corresponds to a case which occurs when an image is taken while the sun is present in the image plane, for example, is generally considered as “a sever condition where generation of ghosts is unavoidable”, and is often permitted. However, for a high class model employing the multistage aperture, it is preferable to reduce ghosts as much as possible even under this sever condition since such an image is often taken by intention.